just hook up a comparator up to it. each time an item drops in, it powers a dispenser which shoots the firework.

And you could still have your hopper hooked up to take the item out.

But imagining that the chest is full (and thus giving out a full signal through a comparator) how would it be able to detect that there is one less item (slightly less signal) and then shoot the firework?

But imagining that the chest is full (and thus giving out a full signal through a comparator) how would it be able to detect that there is one less item (slightly less signal) and then shoot the firework?

By setting the comparator up to a hopper that will only put one item in the chest. You can extend the signal with a repeater that hooks up to a dispenser shooting a firework. Sorry but logic.

But imagining that the chest is full (and thus giving out a full signal through a comparator) how would it be able to detect that there is one less item (slightly less signal) and then shoot the firework?

I had my ideas mixed around a bit. was thinking of a slightly different donation chest. lol

Ok, took me a sec but i got a working version. Its on smp4 8272. I was back feeding the chest with hoppers but that doesnt work. The key was the dropper under the chest to refill it. I will be on later if you need me to break it down for you.

Have a comparator coming out the back of the chest.The comparator powers a block with a torch on it.The torch powers the redstone.The redstone powers the dropper (NOT dispenser).And the dropper puts another dirt into the chest when someone buys one (donates).Now all you have to do is hook up a dispenser with a firework.Also, make sure you keep the dropper full of dirt and that there is always exactly one dirt in the chest.